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Full text of "Practical observations on the generation of statical electricity by the electrical machine"

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PRACTICAL OBSERVATIONS 



ON THE (iEVKRATIOX OF 




?M 



STATICAL ELECTRICITY 



ELECTRICAL MACHINE. 



• 



Lieut. GEORGE W. RAINS, U. S. A., 

Acting Assistant Prof. ot'Clieni., Min. and Geol., U. S. Military Academy. 






7- 



NEW HAVEN: 

PR IN T EI) It Y B . L. HA M I, E N . 



PRACTICAL OBSERVATIONS 



ON THE GENERATION OF 



STATICAL ELECTRICITY 



ELECTRICAL MACHINE 



Lieut. GEORGE W. RAINS, U. S. A., 

Acting Assistant Prof, of Chem., Min. and Geol., U. S. Military Academy. 



X)' 



NEW HAVEN: 

PRINTED BY B. L. IIAMLEN. 
1845. 



PRACTICAL OBSERVATIONS 



Having experienced, at different times, some difficulty in caus- 
ing an abundant and uniform supply of electricity to be generated, 
by the apparatus employed, when the state of the air and other 
circumstances appeared entirely favorable, sufficient inducement 
was offered to devote a few leisure moments to the study of the 
causes of failure. 

In completing a course of experiments with this object in view, 
results have been obtained in the excitation of statical electricity, 
deemed of sufficient value to merit attention. The invention of 
electrical machines being of such long standing, and the subject 
having received the attention of so many eminent persons, the wri- 
ter, with some hesitation, ventures to suggest ideas derived from 
his own observations. The improvements proposed, however, 
being few in number and of simple application, he has thought 
proper to state them, allowing each one who may feel interested, 
an opportunity of satisfying himself of their utility. 

Rubbers. — Commencing with the rubber of the machine, as 
the supposed principal source of failure, it was proposed to ascer- 
tain, first, its mode of action ; second, that substance which would 
be most efficient in its action. 

In regard to its mode of action, the questions which presented 
themselves, were — does it produce electricity by friction, by 
chemical action, or by friction and chemical action ? 

Assuming at first that the effect is produced by friction, its 
mode of action appears to be as follows. The rubber touches the 
glass at any given moment with a certain number of its parts or 
points, and does not, therefore, come into contact with the re- 
mainder ; the friction of these points generates the two electrici- 
ties * of which the positive remains with the glass, and the nega- 



* It is not intended to assume that electricity is one or more fluids, but the ordina- 
ry language is used for convenience. 



live with the rubber. At the succeeding moment, the excited 
parts of the glass have passed opposite to those points of the rubber 
which do not touch ; an inductive action then takes place — the 
positive electricity of the rubber is repelled, and the negative at- 
tracted, by the excited portions of the glass. If the negative elec- 
tricity produced by the first action, has remained with the rubber, 
it will then be in such excess at the points in question, as to force 
itself through the thin intervening stratum of air, and neutralize 
the corresponding positively excited points of glass. 

Hence to remove the negative electricity is of the first impor- 
tance ; and the necessity of being freed from it was perceived in 
the earliest experiments. Sbould this be accomplished, however, 
as perfectly as possible, it is evident that a diminished similar ef- 
fect would necessarily take place, rendering neutral a quantity of 
electricity already excited. What has been observed of the situa- 
tion of the points of the glass and rubber, at the first and second 
consecutive moments of action, evidently continues during the 
entire period of operation. 

From what has preceded, two primary objects are to be attain- 
ed : first, to determine the best method of disposing of the nega- 
tive electricity of the rubber ; second, to obviate the injurious ef- 
fects of those of its parts not in contact with the glass. 

To attain the first object, it naturally suggests itself to conduct 
it off by forming a communication with a conductor between the 
rubber and earth; and consequently, the directions given, and 
carried out in the construction of the best electrical machines, 
have been, to form a metallic connection with the back of the 
rubber, and the surrounding conducting objects. The idea ap- 
pears not to have occurred, that such communication should be 
made with the metallic face, in place of the back of the rubber. 
The cushions being made of non-conducting materials, as silk or 
leather, stuffed with similar substances, the faces of the rubbers 
are insulated, and the negative electricity is prevented from es- 
caping freely. This insulation is so complete in some rubbers, 
that with a twelve-inch plate machine scarcely the smallest quan- 
tity of electricity could be obtained, until the amalgam faces of 
the cushions were connected by a wire with the floor, when its 
amount was suddenly and greatly increased. It has been inter- 



esting to observe the numerous near approximations to this result, 
which, however simple, appears not to have been exactly hitherto 
attained ;— the nearest approach, probably, being the suggestions 
to stuff the rubbers with elastic fragments of metal * and the prop- 
osition to moisten their interior substance.! 

Electrical excitation, in all descriptions of apparatus made use 
of for that purpose, will be increased by adopting the preceding 
suggestion. Should the analgam faces of the rubbers be con- 
nected with the exterior coating of a Leyden jar, in the act of 
being charged, the effect would be much more satisfactory — the 
positive electricity of the coating neutralizing the negative of the 
rubbers. 

The first object having been thus obtained, it remains to pass 
to the second, viz. to obviate the injurious action of those parts of 
the rubber not in contact with the glass. For this purpose, it is 
plain that if a non-conducting substance be interposed between 
them and the excited glass, the desired result will be obtained ; 
and the question resolves itself into the proper manner of applying 
the best substance. If an oxidizable metal or amalgam be em- 
ployed, the oxide which is formed will answer this purpose itself, 
to a certain extent, as the oxides of the metals are imperfect con- 
ductors. For this purpose the oxygen of the air is of service. 
The prominent points in such cases will be kept bright by the 
rubbing of the glass. The oxides thus formed, continue to in- 
crease in most cases, particularly with the amalgams, and ulti- 
mately fill up the intervals; then sustaining the pressure of the 
glass, all farther action of the exciting points of the amalgam 
ceases : in such cases, the surfaces of the amalgam require renew- 
al. The oxides, as will be seen hereafter, are but feeble genera- 
tors. Tallow, lard, and substances of a similar nature, answer 
the purpose much better, and hence they were soon discovered to 
be of service in the earlier researches of electricians. The tallow 
or lard being spread over the surface, or mixed up with the amal- 
gam, surrounded each exciting point of the rubber with a non- 
conducting medium, and hence fulfilled the required conditions. 
As, however, these substances readily combine, mechanically, 



* Am. Jour. Sci. and Arts, Vol. xxiv, p. 256. t Franklin's Letters. 



with the metallic oxides, forming a black, adhesive mass, which 
collects on the glass, soiling its surface, and is troublesome to re- 
move, bees-wax and shellac were substituted, both of which sub- 
stances, when properly applied, answer the purpose remarkably 
well. Neither of them soils the glass, and what is of much im- 
portance, they give rubbers permanent in their action. 

The question now arises, whether there be not other parts of 
the rubber, besides those surrounding the exciting points, which 
may have an injurious effect? That portion which precedes the 
first exciting points at the entrance of the glass, obviously can do 
no harm, as the glass is supposed not to be excited when passing 
in their vicinity ; but the case is materially different with the op- 
posite termination of the rubber, which, not being pressed against 
the glass, is highly injurious — abstracting largely from the elec- 
tricity previously generated. This has also been observed by 
electricians, who do not, however, appear to have proposed any 
substantial remedy; the best, apparently, depends for its success 
on the regularity of the pressure ;* and still another plan is liable 
to the same difficulty.! The silk flap, whose utility appears to 
have been discovered by accident, and whose real object seems 
to have escaped attention, has succeeded or failed, as chance reg- 
ulated its proper or improper application. A certain quantity of 
electricity is doubtless produced by the silk, in whatever manner 
it may be applied, and the amount is considerably increased by 
particles of amalgam which adhere to its surface ; but the total 
quantity thus produced is small compared with that given by the 
rubber, and a larger amount will be obtained by removing the 
flap and increasing the pressure, so as to bring as many new points 
in contact as will equal by their friction that of the silk. Hence 
the above cannot be its true value, neither did its utility appear 
to depend, principally, on its being an interposed non-conducting 
obstacle between the excited glass and the molecules of air ■ 
but rather, having been fastened to the loose edge of the amal- 
gamated leather, this edge was pressed against the glass by the 
adhesion of the silk, and thus prevented from diminishing the 
amount of electricity already evolved. It possesses, also, to some 



Partington's Nat. Phil., p. 151. f Nicholson, Phil. Trans., 1789. 



extent, the power of preventing the electricity of the surface of 
the glass from being drawn off or neutralized by surrounding ob- 
jects. In modern machines, the rubbers are hair-stuffed cushions 
without the loose leather, and the flap is of varnished silk, which 
is so arranged, in some plate machines, as not to touch the glass. 
The first and principal advantage of the silk is thus lost, but the 
second more certainly obtained. 

From what has been stated, it appears to be important to cause 
that edge of the rubber which the glass last leaves in its revolu- 
tions, and which is covered with amalgam, to press constantly* 
and firmly against its surface. This is best secured by making 
use of a leather strip, covered with the amalgam, whose edge in 
question shall be firmly pressed between the glass and rubber, 
and which is consequently narrower than the latter. The idea 
now suggests itself, that in thus contracting the rubbing surface, 
its action may be lessened ; hence, the proper dimensions for the 
rubber are next to be determined. To solve this proposition, re- 
quires a further investigation into the phenomena of excitation. 

Statical electricity has been assumed to be produced by friction, 
and hence the result of molecular disturbance. Taking one of 
the exciting points of the rubber, resting on a corresponding por- 
tion of the glass surface ; suppose such portion of the glass to 
move through an indefinitely small space ; molecular disturbance 
will then be produced, both in the exciting point of the rubber 
and in the corresponding portion of glass surface ; by hypothesis, 
the molecular vibration of the rubber producing negative, and that 
of the glass, positive electricity, each within itself. If it be sup- 
posed that the portion of the surface of the glass, in this indefi- 
nitely small movement, has continued in contact with the exciting 
point of the rubber, then the two electricities, respectively gene- 
rated ill each, will combine or interfere, and a neutral state will be 
the result whilst such contact continues. But should this por- 
tion of glass in its further movement pass beyond the exciting 
point, its molecular vibration continuing for a certain period, will 
evolve an additional quantity of positive electricity, which will 
remain after the molecular vibration has ceased ; and this portion 
of glass will be electrically excited. Should the movement be 
still further continued, and this excited part brought into contact 



8 

with the consecutive exciting point of the rubber, its electricity 
will, by the influence of this point, be nearly if not entirely neu- 
tralized. For otherwise it is difficult, if not impossible, to con- 
ceive of the evolution and absolute contact of the two electrici- 
ties, at such point, without combining or interfering. 

This view of the subject being taken, it follows, that only the 
last exciting points of the rubber produce the effective result. 
This, on first appearance, is a startling conclusion, as it apparently 
reduces the rubber to a mathematical line ; on examination, how- 
• ever, this is found not to be the case ; there must be a certain 
number of these last exciting points, in each of several consecu- 
tive parallel lines, as the points necessarily have spaces between 
themselves ; hence a portion of electricity excited on the glass 
may pass between several, before it emerges entirely from the 
rubber. It follows, therefore, that a certain breadth of rubber is 
necessary, although it must be comparatively small. With rub- 
bers, properly constructed, as will be described hereafter, the max- 
imum effect for the larger machines, was produced by rubbers 
one fourth of an inch in breadth ; and for the smaller, one eighth 
of an inch; the smaller rubbers having generally a greater pres- 
sure. 

To determine the above, as well as to ascertain and confirm 
all other results given in this paper, a numerous set of experi- 
ments was instituted, by means, principally, of three machines. 
One was a glass plate of 12 inches, previously alluded to ; one a 
cylinder of 10 inches diameter and 15 inches long ; and lastly a 
large and beautiful instrument of 38 inches plate, manufactured 
in Paris. To measure accurately the quantity of electricity in 
each case, one of those admirable galvanometers of 3000 turns 
of wire, constructed by M. Goujon of the Polytechnic School, 
Paris, was employed ; the amalgam of the rubbers being connect- 
ed, by means of a copper wire, with one extremity of the coil, 
the other extremity communicating with the prime conductor, by 
means of a glass tube containing water, having a wire inserted 
in each end. The maximum permanent deflection of the needle 
by the 12 inch plate was 16°. 

Having thus attained, satisfactorily, the two objects proposed, 
the discussion will be taken up on the questions first suggested, 



viz. is statical electricity produced by friction, by chemical action, 
or by friction and chemical action ? The solution of the first 
two solves the third. To this branch of the subject, it was not 
considered necessary to devote much attention ; for the results 
obtained by others, superior to the writer in abilities, have decided 
quite conclusively, that chemical action does not produce the ex- 
citement of the electrical machine. Indeed it is difficult to con- 
ceive, how ordinary chemical action in the amalgam of the rub- 
bers, can influence the generation of electricity, except in the single 
case previously mentioned. For if it be supposed, that the sur- 
face of the amalgam is made up of numerous small galvanic cir- 
cles, as is doubtless the case, the air acting, possibly in conjunc- 
tion with its watery vapor, as the exciting medium ; no action 
could be produced, unless such circles, either singly or collective- 
ly, were closed. Under the improbable supposition, that parts of 
the glass acted as conductors to complete such circles, it would 
be contrary to all analogy to suppose, that in their movement, 
they carry off a portion of such current. Neither can it be sup- 
posed, that it acts by inductive influence; as in such case, the 
induced electricity would be of a tension, so extremely low, as 
not to be apppreciable. 

In order, nevertheless, to satisfy any existing doubt, a tube 
electrical machine was constructed,* whose piston performed the 
part of a rubber ; and the apparatus arranged, so as to admit of 
being filled with different gases. It was thus found that air, oxy- 
gen, nitrogen, and carbonic acid gases, when thoroughly dry (and 
a vacuum) produced nearly the same amount of electrical action, 
when the same oxidizable or non-oxidizable rubbers were em- 
ployed ; hence this result coincides with that obtained by others.f 
The conclusion is therefore adopted, that the electrical machine 
produces its effect entirely by friction. 

The second branch of the subject will now be examined, viz. 
to ascertain what substance is most effective, in generating elec- 
tricity by friction. In the endeavor to attain this point, numer- 

* American Journal of Science and Arts, Vol. xxvi, page 111. 

1 Dans la production de I'electricite par frottement, Taction de l'air sur lea en- 
duits, plus ou moins oxidables des frottoirs, ne parait exercer aucune influence sur 
les effets 61ectriques qui eu resultent. — Pedet's Mtmoirs. 

2 



10 



ous experiments were made with various substances : a list of 
some of them is given, arranged according to their effective ac- 
tions. 
1. BisulphuretofTinandamal-20. Bismuth. 



gam. 

2. Common Amalgam. 

3. Pure Mercury. 

4. Bisulphuret of Tin. 

5. Tin foil. 

6. Zinc filings, (fine.) 

7. Copper filings, (fine.) 

8. Silver. 

9. Gold. 

10. Platina. 

11. Lead. 

12. Caoutchouc. 

13. Silk. 

14. Paper. 

15. Leather, (soft.) 

16. Woolen. 

17. Plumbago. 

18. Iron filings. 

19. Antimony. 



21. Galena. 

22. Talc. 

23. Chromate of Iron. 

24. Protoxide of Copper. 

25. Protosulphuret of Mercury. 

26. Chromate of Lead. 

27. Protoxide of Bismuth. 

28. Peroxide of Manganese. 

29. Peroxide of Mercury. 

30. Protoxide of Zinc. 

3 1. Protoxide of Mercury, 

32. Protoxide of Tin. 

33. Shellac. 

34. Wax, (no action.) 

35. Tallow, " 

36. Lard, " 

37. Bisulphuret of Mercury with 
lime, gives negative elec- 



tricity. 

From the preceding, it appears that bisulphuret of tin rubbed 
over a surface of amalgam, containing but little mercury, is the 
most efficient of all substances employed ; it is, however, inferior 
in value to the common amalgam, on account of its transient ac- 
tion, requiring frequent renewal ; and the quantity of electricity 
evolved, soon being but little more than that capable of being 
produced by the amalgam itself. The amalgam, in case the sul- 
phuret is employed, acts principally, by serving as a metallic com- 
munication to convey off the negative electricity, as rapidly as 
generated. Tin or copper filings answer the same purpose, 
nearly as well as the amalgam. 

Hence the common amalgam has been selected, as the most 
suitable material, on account of the quantity of electricity pro- 
duced, as well as its ease of application ; and, when properly ap- 
plied, the valuable steadiness of its action. Its composition is but 



11 

of little importance, equal parts, by weight, of zinc, tin, and mer- 
cury, answering every purpose. The zinc and tin are to be melt- 
ed together, the mercury then added, and the melted mass pour- 
ed into a wooden box, and agitated violently until cool ; then it 
is to be still further reduced to a fine powder, by being rubbed in 
a mortar. The various results obtained by different electricians, 
each recommending a new proportion of ingredients, appear to 
have been caused by the different conducting powers of the cush- 
ions of the rubbers employed ; they having failed, probably, in 
each case to connect the metallic faces with the earth. 

It will be observed that the oxides of zinc, tin and mercury, 
yield but a small comparative quantity of electricity; hence the 
necessity of frequently renewing the amalgam of the rubbers, 
constructed after the ordinary method. The combination of mer- 
cury with the common metals, being rapidly oxidized by reason 
of the galvanic action, shows the reason why "amalgams con- 
taining much mercury are of transient and variable action."* 

It is probable that pure mercury, if it were possible to apply it, 
so as to cause as much friction between its particles and the sur- 
face of glass as takes place with other metals, would surpass all 
other substances in its effective capabilities. This, however, is 
impossible as long as it continues fluid, on account of the mobility 
of its particles; and this mobility constitutes its chief value, by 
allowing a more perfect contact with the glass; hence its maxi- 
mum effect can be approximated to, only by rendering it semi- 
fluid, in forming an amalgam. 

The number of rubbers to be employed demands some atten- 
tion, and at the same time the action of the double rubbers, of 
the plate machines, will be examined. Theoretically, the num- 
ber of rubbers is unlimited ; for if one produces a certain effect, 
six would produce six times that effect, if the electricity be re- 
moved as rapidly as evolved ; but in practice, the number is neces- 
sarily limited, and this limit depends, collaterally, on the size of 
the plate or cylinder, and the convenience of construction. Cyl- 
inder machines have but one rubber, which arrangement may 
have had its origin, in the larger machines, merely from the 

* Singer's Treatise on Electricity, page 52. 



12 

slightly increased difficulty of construction. This arrangement 
necessarily lessens their power one half. 

Plate machines have generally two pairs of rnhbers, or four in 
all ; in large plates, this number might be increased to three or 
four pairs, with corresponding increase of power ; but the labor 
of working the machine would increase in the same ratio ; which, 
in this kind of machines, is a material circumstance. However, 
with rubbers constructed after a manner to be described, the labor 
caused by two rubbers, is but little greater than that caused by 
one, of the common construction. 

The action of double rubbers will now be discussed. A single 
rubber evolves a certain quantity of electricity ; one surface of 
the plate being thus excited, the inductive influence causes a cer- 
tain amount of positive electricity to become free on the opposite 
face; which acts also by its induction on that of the opposite 
surface, and increases its amount ; and this action and reaction 
between the surfaces, continue until an equilibrium is established ; 
the result being, that the original electricity generated, is nearly 
doubled in its amount. If in this condition, the positive induced 
electricity of the second surface be removed, it will leave the 
corresponding quantity of negative electricity on this surface of 
the glass, which will neutralize the opposite positive surface ; and 
nearly all signs of excitation, on such surface, will consequently 
cease. By continuing this process, the second surface of the glass 
gradually ceases to give off electricity ; and the quantity generated 
on the first surface, not being increased by induction, becomes 
comparatively feeble in its action. The plate has now become 
charged, in a manner similar to that in a Leyden jar ; and if re- 
moved and placed on a ring of metal, a corresponding ring being 
then placed upon it, and opposite to the first, by forming a con- 
nection between the two, a strong discharge will take place, and 
the plate resumes its first condition. 

From the preceding, the following conclusions are drawn : 1st, 
the quantity of electricity generated by the rubber, has its amount 
nearly doubled by inductive action ; 2d, to maintain this in- 
creased effect, it is necessary that the induced electricity of the 
second surface be allowed to remain on that surface. The in- 
ductive action being a decreasing function of the distance, it 



13 

follows, that the thinner the glass, the greater will be the effect ; 
which shows the value of the first conclusion. From the second, 
it appears, that if a cylinder have a damp interior surface, its ef- 
fective action will be much diminished ; all cylinders should, 
therefore, have their interior surface perfectly dried, and then be 
sealed up air tight. If varnished with shellac, which has not 
been colored by the addition of any other substance, the cylinder, 
after having been once dried, may remain open without material 
injury to its generating powers. 

The machine being in action with one rubber, it will be 
supposed that a similar one may be arranged to the second sur- 
face of the glass, and diametrically opposite to the first rubber. 
A certain quantity of positive electricity being generated by the 
second rubber, it will find itself in contact, and will unite with, 
the similar electricity induced on that surface of the glass, by 
the action of the first rubber. Hence the amount on this sur- 
face will be much increased ; and this additional quantity, acting 
inductively on the first surface, will likewise increase its excita- 
tion ; the final result being, that the electricity produced by the 
rubbers, on each surface of glass, is nearly doubled by inductive 
influence. 

It may at first appear, that by arranging points to each surface 
of the glass, nearly four times the quantity evolved by one rub- 
ber might be collected ; on a closer examination, however, it will 
appear that but little more than one half of this amount can be 
rendered available ; for as soon as the free electricity of one sur- 
face is removed, that of the other, to a great extent, becomes ne- 
cessarily neutralized. The plate acquiring a charge, similar to 
that produced in the experiment of the single rubber, although 
to a much less extent ; and as the plate revolves, passing between 
the connected metallic faces of the rubbers, this small charge is 
neutralized. 

It has been so far assumed, that the rubbers were placed dia- 
metrically opposite ; allowing one to retain its situation, the oth- 
er will be supposed to change its position. A certain quantity of 
induced positive electricity being evolved on the second surface 
of the glass, by the action of the first rubber as already discuss- 
ed ; this passes with the glass surface in its revolution, to the me- 



14 

tallic face of the second rubber, where it necessarily becomes ab» 
sorbed by the negative electricity of that rubber, and the free 
positive electricity, generated by the first rubber on the first sur- 
face, is thus, to a great extent, rendered neutral. If the metallic 
faces of the two rubbers be in connection, as is the supposition, 
the glass surfaces will thus be brought back nearly to their primi- 
tive state, and the product of the action of the first rubber ceases 
almost entirely to exist. 

It follows from this, that it is important that the rubbers be 
placed opposite to each other ; a slight variation is not, however, 
very perceptible, by reason of the vibrating molecules of the 
glass, continuing to produce positive electricity, after passing to 
a certain distance the exciting points of the rubber. That this 
is a correct hypothesis may be shown by the following experi- 
ment : let the back of the hand be held near the second surface 
of the glass plate of the machine ; on rubbing the opposite sur- 
face with a piece of silk, every motion of the rubber, will be 
distinctly and instantly perceived. 

Hence, if the electricity of the surfaces ceases to be generated, 
when the portions of glass pass beyond the exciting points of the 
rubbers, it follows, that if one of the rubbers be somewhat in ad- 
vance of the other, the electricity induced by the remaining rub- 
ber, will be mostly absorbed, and the practical action of this rub- 
ber destroyed. 

The length of the rubbers will now be determined. In cyl- 
inder machines, the rubbers should extend so as to rub the en- 
tire exposed surface of glass ; the ordinary practice of limiting 
their length to a portion of the surface, appears to be deficient in 
principle. For, let it be supposed that such is the case ; then 
those parts of the exposed glass surface, not subjected to the rub- 
bing action, hence not yielding electricity, must conduct off a 
small quantity of that produced elsewhere. But should the rub- 
bers extend to the axis, an additional quantity will be evolved 
and if the axis abstract a portion, on account of its proximity, it 
will be but a small part of this additional quantity. It cannot 
be supposed, that in exciting those portions of the glass near the 
axis, the glass itself becomes a better conductor. 



15 

It is important that the elements of the glass, subjected to the 
action of the rubbers, be as long as possible ; for the glass surface 
may be considered as composed of an indefinite number of con- 
secutive portions, and as each one of these portions, when excited, 
acts inductively on those around it, it follows, that the greater 
the number excited at the same time, the greater must be the re- 
ciprocal inductive influence. Hence large machines, when un- 
der the same circumstances, must give electricity of a higher 
tension, than that produced by smaller machines. 

Having thus discussed the principles of action, of the various 
parts of the rubber, it remains to apply them practically. To 
obtain the narrow strip of rubber, requires the following process : 
being provided with a strip of common amalgamated leather, 
subject it to strong pressure, in order to render the surface flat and 
smooth ; it is then to be rubbed with a clean cloth, to remove 
any excess of lard or tallow, if such substances be employed;* 
which must still further be removed from the exciting points, 
by rubbing the amalgam with a piece of smooth leather dipped 
into mercury, to which a few particles will adhere ; this not only 
cleans the exciting points, but enlarges their extent. 

If the amalgam be now applied to the glass with pressure, 
those portions of the surface still covered with the lard or tallow, 
might come into contact at some points with the surface, and 
produce a detrimental effect ; to avoid this, reduce some plum- 
bago (black lead) to a fine powder, which is to be rubbed over 
the surface of the amalgam, and by adhering to such portions 
will render them excitable points. It also permits greater press- 
ure for the same amount of friction ; and thus the surface is 
brought more intimately into contact with the glass. From 
the leather prepared as above described, a strip one fourth of an 
inch broad, and somewhat longer than the rubber, is to be cut; 
carefully avoiding to break up the amalgam at the edges, which 
may be accomplished by covering it with pasteboard and cutting 
through both substances. 

The rubber proper being prepared as above directed, it remains 
to apply it to the cushion, which, to avoid unnecessary resistance, 

* Wax and shellac, as previously stated, may be used with advantage, but they 
require more care in the manipulation. 



16 

should be three quarters of an inch or one inch in breadth, and 
this last dimension should not be exceeded in the largest machines. 
The cushions should be made quite firm, and not stuffed with 
hair, as that does not allow them to offer sufficient resistance ; 
they should be as perfect non-conductors as possible ; the backs 
of the cushions should be of glass or well-baked wood, in order to 
prevent the cushion from abstracting a portion of the electricity- 
generated by the rubber. A piece of smooth white leather, about 
three times the breadth of the cushion, should be fastened by 
one of its edges to the glass or wooden back of the rubber, and 
passing over the face of the cushion, will thus be pressed against 
the glass, the remaining edge being loose. This flap of leather 
should not be very thin, otherwise there will be a useless adhe- 
sion to the glass, increasing the amount of friction unnecessari- 
ly ; it should be kept perfectly dry, as the entire action of the 
machine, be it ever so powerful, will be destroyed should it be- 
come damp on the surface opposed to the glass. To this leather 
flap, and opposite to the centre line of the cushion, fasten the 
strip of amalgamated leather by means of a little warm bees- wax ; 
the face of the leather will then exhibit the appearance as shown 
by the adjoining figure: the projecting end of the rubber proper 
being left, for the purpose of forming a metallic com- 
nication between it and the exterior coating of a Ley- 
den jar, or with the earth. 

To obtain the maximum effect, those portions of 
the leather flap on each side of the amalgam strip, 
which are pressed against the glass by the cushion, 
should be touched with a little bisulphuret of tin, 
which, in the larger machines, will be found to increase 
powerfully the action ; it requires to be renewed, how- 
ever, every twenty minutes, whilst the machine is being worked ; 
at each of which renewals, the amalgam should be wiped clean, as 
the sulphuret of mercury, which may be formed, is detrimental 
in its action. It may be well to observe, in this place, that the 
glass should be oiled and wiped clean before a course of experi- 
ments, to prevent its surface from attracting moisture. 

The rubber being finished, it must be thoroughly dried to ex- 
pel any moisture, before being applied to the machine. The ordi- 



l^s 



17 

nary degree of firm pressure, employed in the common-sized ma- 
chines turned by means of a crank, has been found to produce 
nearly the maximum degree of excitement ; any increase of pres- 
sure above this limit, although generally followed by an increas- 
ed quantity of electricity, is inexpedient, as the additional amount 
thus evolved, does not compare with the increase of friction ; 
between zero of pressure and this limit, however, the quantity 
generated is directly proportional to the friction or pressure. 

Large plate machines, as will be seen hereafter, admit of less 
pressure on equal surfaces than cylinders, and consequently are 
less effective, as the limit above alluded to is not attained. 

Plates and Cylinders. — It is now proposed to examine into 
the comparative qualities of plates and cylinders, in order to de- 
termine their relative values as electrical generators. 

If an unpolished glass surface be employed, with an amalgam 
rubber, the electricity generated on the glass will be negative; if 
it be partly rough and partly polished, the rough parts will give 
out negative, and the polished positive electricity ; if equal in 
extent, they will neutralize each other's action, and no effective 
result will be obtained. As the polished surface increases in ex- 
tent, the rough surface decreasing in the same ratio, the positive 
electricity acquiring the ascendancy, will increase in quantity 
until the glass becomes entirely polished, when its intensity will 
be at a maximum. It follows from this, that the fineness of the 
polish is a necessary qualification. Inequalities in the surface 
lessen the effect, by reason of the depressions not being acted 
upon properly by the rubber ; hence the surface should be ground 
smooth before polishing. The glass should be kept perfectly 
clean ; as any substance adhering to its surface would cause the 
rubber to act on that in place of the glass, and by hypothesis, a 
diminished result would be obtained. Should such substance be 
taken from the rubber, it carries with it a portion of its negative 
electricity, thus neutralizing a portion of the positive surface * 



* Quelorsque l'un des corps soumis a l'expcrience est entame par 1'autre, celui- 
ci, outre, l'electricite qui lui est propre, prend encore, avec la petite couche mince 
de la substance qu'il enleve, une portion d'electricite propre a cette dermere ; de 
sorte que la sienne se trouvant modifiee, peutetre positive, nulle ou negative.— M. 
Becquerel, Vol. II, p. 122. 

3 



18 

From what precedes it appears, that plates have the advantage 
of admitting a finer polish and more uniform surface than cylin- 
ders ; which latter, however, are usually thinner, and therefore 
the inductive action is stronger. It will be supposed that the 
advantages of each are in these respects balanced ; it remains to 
compare their powers. For this purpose, a plate of thirty six 
inches diameter, with four rubbers twelve inches long each, will 
be compared to a cylinder of twelve inches diameter, and eigh- 
teen inches long, with two rubbers eighteen inches long each. 
As the velocity of the portions of the glass passing under the 
rubbers is an increasing function of their respective distances 
from the axis of motion, the velocity of the circumference of that 
circle, which touches the centres of the cushions, will be taken 
for the mean; and this, multiplied into the total length of rub- 
ber, will represent the amount of glass surface subjected to the 
rubbing action for one revolution. Hence, 

Inchos. 

(37-6992)48 = 1S09-5616 for one half revolution of the plate. 

(37 0992)36 = 13571712 for one revolution of the cylinder. 

It is assumed that the amount of force expended in each ma- 
chine for each unit of time is equal ; hence but one half of a 
revolution of the plate is considered; for the diameter of its 
mean circle of resistance being twice the diameter of the cylin- 
der, it follows that the plate will make but one half of a revolu- 
tion, whilst the cylinder performs one entire revolution. Friction 
being directly proportional to pressure, it is evident that the sum 
of the pressures in each machine must be equal : hence the 
same amount of pressure is exerted on forty eight inches of rub- 
ber in one case as is applied to thirty six inches in the other; an 
inch of each is then pressed in the inverse ratio of these num- 
bers, or as 3 to 4. But by hypothesis, the greater pressure pro- 
duces the maximum effect ; hence each inch of the plate rub- 
bers does not exert its greatest action ; and as it has been assum- 
ed, that up to the maximum pressure for the same extent of sur- 
face the disengagement of electricity is directly proportional to 
the friction, it follows that the quantity given out by each inch 
of the rubbers of the plate, is to the quantity given out by each 
inch of the rubbers of the cylinder, as 3 to 4. But each of the 



19 

rubbers of the same machine produces, by hypothesis, an equal 
effect on each equal portion of glass surface subjected to its ac- 
tion ; hence is obtained, for the total effective action for a unit of 
time of each machine, as follows : 

(1809-56 16)3 = 54286848 for the plate machine. 
( 1357-1712)4 = 5428-6848 " " cylinder " 

The machines are therefore equal in power. This result has 
been confirmed by accurate experiment. It is conceived that the 
variety of opinions expressed on this subject has proceeded from 
the use of but one rubber to the cylinder, and from inattention 
to the proper method of carrying out the details of construction. 
It results from the foregoing discussion, that for large machines 
the cylinders are much to be preferred, for economy of construc- 
tion, occupying less space, being less liable to accidents, and for 
the convenient collection of the negative electricity. Plates are 
preferable for small machines, by reason of being more compact 
as well as of finer appearance, and on account of the interfering 
action of the points of the prime conductor, which emit sparks 
to the rubbers if too nearly approximated. 

Cylinder machines with two rubbers being thus found superior 
in many respects, when large machines are required, the follow- 
ing representation is given for reference of construction. The cyl- 
inder (A) is twelve inches in diameter and eighteen inches long ; 
it is supported by two pairs of glass pillars, (B)(B), (B)(B), one 
and a quarter inches in diameter each, and three feet long ; or of 
one half this length, and joined together at the axis of the cylin- 
der by brass tubes four inches in length ; these tubes being con- 
nected by a cross piece, furnish supports to the axis which turns 
between the glass pillars; these are placed one inch apart. 

The rubbers (C) (C) have glass backs one inch broad, and one 
and a half inches deep, and are about two feet long, moving be- 
tween the glass pillars, which, by means of brass caps or sockets 
and screws, cause the rubbers to maintain the proper degree of 
pressure. 

The positive prime conductor (P)(P), is composed of two 
branches, one on each side of the cylinder, each of which is four 
inches in diameter, and three feet long ; these are joined at their 
farther extremities by a cross tube of two inches in diameter, 



20 



which has a branch one inch in diameter, and six inches long, 
terminated by a ball two inches in diameter. The cross piece 
should be movable in the sockets of the prime conductor. 




The negative prime conductor (N)(N) is four inches in diame- 
ter, and three feet six inches long, supported by the two pairs of 
glass pillars; it is connected to the top rubber by means of a 
brass rod (R) one half inch in diameter, which is loosely insert- 
ed in a hole in the rubber which communicates with the amal- 
gam. The rod can be withdrawn, and by this means the upper 
conductor becomes insulated from the rubber, and may be con- 
nected to the positive prime conductor, of which it will then 
form a part. The latter conductor is supported by four glass pil- 
lars, (G)(G)(G) (G), eighteen inches long each. The amalgam 
of the lower rubber communicates with a Leyden jar or the ta- 
ble by a chain or wire (W), which also is connected to the upper 
rubber when the negative electricity is not wanted.* The crank 

* By applying a detached row of points communicating with the ground, and 
near to the glass between (A) and (C), both conductors will be charged at the 
same time, — one with positive and the other with negative electricity. 



21 

(T) gives the motion to the cylinder. The many advantages of 
a machine arranged in this manner become evident on reflection ; 
it is sufficient in this place to observe, that if made after the 
manner directed, it will equal in power two large plate machines 
(the diameter of each plate being three feet) constructed after the 
common method, and using the ordinary rubbers. 

Prime Conductors. — Having already extended this paper be- 
yond the original intention, the remainder of the subject will be 
concisely treated. Prime conductors of the ordinary form should 
be of such size as to hold on their surfaces electricity of the same 
tension as that of the glass, without throwing any of it off. 
" This tension cannot be exceeded."* If of greater size, they 
are injurious by reason of their increased surface, presenting an 
increased extent to the conducting action of the air. If of small 
size, unless spherical, the excess of electricity is rapidly thrown 
off; in charging an electrical battery, however, they are prefera- 
ble to those of larger size. 

Prime conductors probably have the best form for common 
purposes when they are composed of two branches, each having 
a length double that of the cylinder, (or equal to the diameter of 
the plate,) and a diameter one fourth that of the cylinder, or one 
tenth that of the plate : connected by a cross piece one half the 
diameter of the conductors. Brass smoothly gilt is the best 
ordinary material ; it should be neither varnished nor painted, 
for the sparks break through such coatings, leaving rough points, 
which are sometimes highly detrimental.! 

The only part requiring particular attention on the subject of 
the prime conductors, is their points to receive the electricity of 
the glass. A common pin is about one inch long and one twen- 
tieth of an inch in diameter ; let it be supposed that both ends 
are pointed and covered with little balls of wax; apply it to an 
excited body; it will receive a certain charge which has a ten- 
dency to escape, which tendency, for a unit of surface equal to 
that of the point of the pin, at the central portions, will be repre- 
sented by unity. Remove the wax balls; the pin may now be 
considered a prolate spheroid, whose transverse axis is one inch 

* M. Becquerel, Vol. II, p, 205. 

t Faraday's Chem. Manip., note by Prof. Mitchell, p. 452. 



22 

in length, and its conjugate, one twentieth of an inch. But in 
such case the tendency to escape at the central portions of the 
ellipsoid, is to the same effort at the extremities, " as the square 
of the conjugate axis (1 ? ) is to the square of the transverse 
(20 2 ):"* hence the effort to escape at the point of the pin is 
represented by 400. Let it be now supposed that the pin retains 
its point but doubles its own diameter ; the proportion will then 
be as 2 2 to 20 2 , or as 1 to 100. Hence by doubling the diame- 
ter of the pin, it has diminished the power of its point to one 
fourth ; this shows the importance of having slender points. 

The influence of a point depends moreover on the tension of 
the electricity, and appears to act as follows. From the position 
at which the point first shows signs of being acted upon, draw a 
cone of rays tangent to the exciting electrical atmosphere, having 
the point at the vertex ; this cone of influence being formed of 
neutralizing rays, the intensity of their action, by the laws of 
induction, depends on the distance of the point from the exciting 
body. As the point approaches this body the elements of the 
cone, remaining tangent, diverge until having reached a certain 
degree of divergency depending on the intensity of the electrical 
action, they cease to separate ; and if the point continue to ap- 
proach the excited body, the cone will be intersected by this 
body. These intersections decrease in extent until the point 
touches the body, when its influence, except for the correspond- 
ing point in contact, ceases. For electricity of low tension, the 
point being that of a common sized needle, the limiting angle 
of the elements of the cone appears to be about 166°, which, if 
the point be at one fourth of an inch distant from a plane exci- 
ting surface, will intersect such surface in a circle, whose diam- 
eter is about four inches. The electricity within the circumfer- 
ence of this circle will be entirely neutralized. It appears there- 
fore that the points of the prime conductor to collect electricity 
of low tension, should be needles, and placed not farther apart 
than four inches. The electricity on the glass surface on leaving 
the rubber, being of high tension, soon commences to be acted 
upon by the points of the conductor ; its tension rapidly dimin- 



* Murphy's Mathematical Discussions on Electricity, p. 69. 



23 

ishes as it approaches the points, and when opposite, entirely 
ceases. The prime conductor however being insulated, and hav- 
ing acquired a certain degree of tension itself, refuses to accu- 
mulate any more electricity of the same or lower tension ; the 
parts of the glass opposite to the points being thus in an excited 
condition, electricity of higher tension arrives nearer and nearer 
to the point at each revolution ; its inductive action causes the 
elements of the cone of influence again to diverge, enlarging the 
area of the intersection. The prime conductor and glass having 
arrived at the same electrical state, those points which find them- 
selves in the most favorable positions, will receive the electricity 
having the highest tension, and the remaining points, in place of 
receiving, will give off electricity to those portions of the glass 
which may, from any defect, have less tension. It hence appears, 
that needle points should not be nearer to each other than four 
inches, to collect electricity of the lowest tension : as this how- 
ever in most cases is almost instantly increased, five inches would 
answer a better purpose, in order to diminish the number as 
much as possible. In charging large batteries, an additional set 
of movable points might be employed, to be taken off as soon 
as the tension reached a certain extent. From the discussion of 
the action of the rubbers in plate machines, it will be concluded, 
necessarily, that but one permanent set of points to each double 
rubber should be employed, which may be on either side of the 
plate — a set for each surface being not only useless but injurious. 

By applying the suggestions given in this paper to electrical 
machines of the common construction, it will be found as a 
general result, that those of the best construction will double 
their action, and that others will more than quadruple the 
amounts previously generated. 

Thus has the subject of the ordinary excitation of statical elec- 
tricity been as fully discussed and applied, as the small amount 
of leisure time at the disposal of the writer would admit ; and 
although necessarily imperfect, it is still confidently believed to 
contain hints, which if acted on, will richly repay the electrical 
amateur.